Email marketing to your company's customer list is one of the most overlooked avenues to consistent results and revenue–bar none. I think there are a few reasons for this.

The first is the stigma of spam–we don't want to annoy our customers; we all loathe bloated inboxes stuffed with email promotions.

The second is that unlike SEO (where you can see intuitively the value of being No. 1 for a particular keyword) or pay-per-click marketing, which has hard media costs associated with it, it is very hard to understand the difference between where your email marketing campaign is now and how good it could be.

The third is that "email marketing" is nearly always one of a dozen bullet points in a marketing person's job description. And because of the above two issues, it stays at or near the bottom of that list.

Getting It Right

At Wpromote, we spent years failing to do email well–for, basically, all of the above reasons–before we finally took the plunge and started providing email marketing management for our clients.

Here are the six biggest things we have learned so far.

Email more often ... The vast majority of email marketing programs consist of a monthly newsletter and sporadic sale or new product announcement emails. You need to take off your "I hate my inbox" hat and email your users more. Once a week is a minimum–and for most campaigns, we email two or more times per week.

... But change it up! Make sure that your emails are fresh and that the offers and messages vary. Bonobos is one of my favorite clothing companies and has an aggressive email campaign (I seem to get an email every other day). But they are fun, witty and fresh–so that even though the vast majority of the emails go straight into the trash, I do not unsubscribe. And when the time is right for a new pair of pants, they are top of mind.

Use "remails." Constantly coming up with new emails can be exhausting–but you can sometimes double your results simply by what we call "remailing." For every email you send, resend it later to those that didn't open the email–this time with a radically different subject line. (Nearly all email service providers allow you to segment this way.) This could offer a 20% to 100% increase in results, with just a few minutes of work.

Make it compatible. The number of devices, programs and websites that your emails are viewed in is staggering. So make sure that you are compatible with desktop software (like Outlook), Web-based email (like Gmail), and mobile devices, of course. One of the biggest mistakes: Because so many email programs disable images by default, make sure that your email is readable without the images showing. We use Litmus.com to check how our emails look in dozens of devices and email clients; you send it an email, it spits back screenshots of how it looks nearly everywhere.

Have a clear call to action. Very much like a website landing page, each email you send should have a clear, primary goal and call to action–one single thing you want every user to do. This is not to say that there aren't lots of other "things to do" on the email, for those users that choose to keep scrolling down; we love to include bestsellers, main categories, and so forth. But the "above the fold" real estate is perhaps even more dramatic on email–so make sure to get to a single point, and do so quickly.

Don't forget your bestsellers. We have one large retail client that absolutely loved to focus its email content on new products and styles. Its staff assumed that existing clients already knew about the main product line–and the people sending the emails may have grown bored themselves of covering the same products. But guess what? When we refocused the company's emails on its core and best-known products, the revenue from email marketing skyrocketed.

Email may be boring, it can be annoying, and it certainly is not the new kid on the block. But it is a consistent revenue driver with an unmatched ROI–and can be a boon to businesses new and old.